I've seen quite few real reviews of KDE 4.2 so far. Let me show you a screen shot and begin with my summary: It's a great release.

PlasmoidsThe core, no actually every part of the KDE4 desktop consists of plasmoids, little applets comprising parts of the desktop: The menu, the panel, the clock, desktop applets such as wheather and disk space, network and CPU usage displays, news overviews and many more are already included.

After some problems with achieving the high goals of a flexible interface the developers have by now achieved much. You can put the plamoids anywhere on your screens. I have two displays and really enjoy being able to setup each display with its corresponding plasmoids. I really enjoy the prettier new taskbar and the new style. You can now move plasmoids from the panel to the desktop and back, but that's not easy yet.

PlatformsI don't know about the OS X port, but my girl friend is already enjoying the Windows port of KDE including great applications such as Parley in a native Windows build. And they work more or less flawlessly. I think the sounds support was not perfect, but everything else works nicely and the installer is pretty simple to handle while remaining flexible. (It's not easy enough yet that she could install it herself, though.)

StabilityAs it's a .0 release, you will definitely be able to crash plasma. Probably during the tryouts for setting up your plasmoids. I recommend quitting and restarting plasma after you've got your basic configuration done with kquitapp plasma && plasma &. Then continue with the fine tuning.

UpgradeKDE used 4.2 my previous(4.1.4) configuration successfully. Even my applets were still there at the same place. That's a nice thing and I think between 4.0 and 4.1 it didn't work well for me.

BugsThe screen flickering problem disappeared for me. In KDE 4.1 my second screen kept going blank for about 1,5 seconds about every 10 seconds. I had to work around that bug by starting kde manually. That's solved now. I get one flicker during that startup of KDE but that's it.

PolishThere will of course be some polish in the 4.2.x releases. Some plasmoids are still buggy and can crash plasma. The weather applet crashed plasma if it didn't find the city I was searching for e.g. Sometimes removing an applet can crash plasma. But there are more practical plasmoids and they work better than in 4.1. The rss reader lets you scroll through with your mouse wheel, a memory usage applet is still missing.

EfficiencyYou can see how efficient the plasma approach and implementation is when suspending your computer and then resuming it. The clock will take up to a minute to be set to the current time. That's because it gets the time only once a minute - when a new minute starts. That saves CPU usage in between and shows how much thought the programmers put into an efficient implementation. Even my desktop full of plasmoids uses only very little resources of my system. It doesn't slow you down. It even works nicely over NX (remote desktop). When idling, plasma creates about 1 wake per second in powertop on my system.